Glucose aldehyde structure

Jl exists in this form only in solution, though stable derivatives of the aldehyde structure are known. The optical antipode of D-glucose in which the positions of every H and OH are transposed is L-glucose. 

The chemical properties of monosaccharides are further complicated by the fact that they can exhibit tautomerism in aqueous basic solutions (Figure 1.15). This means that after a short time a basic aqueous solution of a monosaccharide will also contain a mixture of monosaccharides that will exhibit their characteristic chemical properties. For example, a solution of fructose will produce a silver mirror when treated with an ammoniacal solution of silver nitrate (Tol-len s reagent). This is because under basic conditions fructose undergoes tautomerism to glucose, whose structure contains an aldehyde group, which reduces Tollen s reagent to metallic silver. 

If all the hydroxyl groups are converted into —O.CH3 by methylation, the two end glucose units will each have four methoxy groups compared with three in all the other components in the chain. When the methylated cellulose is hydrolysed, B above breaks down with the loss of one methyl group. There will therefore only be one tetramethyl glucose unit in the product of hydrolysis, and this can be separated by distillation under reduced pressure. There has, however, been some dispute about the accuracy of this method. More recently Wolfram J. Amer. Chetn. Soc. 1937, 1938, 1939) formed mercaptals with the aldehyde structural isomer... 

FIGURE 22.4 Haworth formulas for the cyclic hemiacetal forms of D-(+)-glucose and their relation to the open-chain polyhydroxy aldehyde structure. (Reprinted with... 

Fischer had already developed a method for effectively interchanging the two end groups (aldehyde and primary alcohol) of an aldose chain. And, with brilliant logic, Fischer realized that if (-E)-glucose had structure 4, an interchange of end groups would yield the same aldohexose ... 

The sugars with the formula C6H12O6 known in 1886 were glucose, fructose, galactose, and sorbose. Of the known hexoses, two types of structures were present. These types were the glucose-galactose type with aldehyde structures and the fructose-sorbose type with ketone structures. 

Figure 22.4 Haworth formulas for the cyclic hemiacetal forms of D-(+)-glucose and their relation to the open-chain polyhydroxy aldehyde structure. (Reprinted with permission of John Wiley Sons, Inc., from Holum, J. R., Organic Chemistry A Brief Course, p. 316. Copyright 1975.)...

Aldose-ketose isomerism Fructose has the same molecular formula as glucose but differs in its structural formula, since there is a potential keto group in position 2, the anomeric carbon of fmctose (Figures 13 and 13-7), whereas there is a potential aldehyde group in position 1, the anomeric carbon of glucose (Figures 13-2 and 13-6). 

In contrast to other 2,5-anhydroaldoses (which exhibit mutarota-tion, possibly due to the formation of hemiacetals28), 2,5-anhydro-D-glucose does not show any mutarotation.27 The importance of this compound as a potentially useful precursor to C-nucleosides warrants a reinvestigation of the deamination reaction, and the definitive proof of the structure of the compound. The readily accessible 2,5-anhydro-D-mannose (11) does not possess the cis-disposed side-chains at C-2 and C-5 that would be required of a synthetic precursor to the naturally occurring C-nucleosides, with the exception of a-pyrazomycin (8). The possibility of an inversion of the orientation of the aldehyde group in 11 by equilibration under basic conditions could be considered. 

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

---